Metallogenesis Of Cobalt Deposits In Eastern Kunlun Orogenic Belt,Qinghai Province

As an important component of the central orogenic belt,the East Kunlun orogenic belt has undergone multiple tectonic evolutions,and retaining geological information about the evolution of the Precambrian supercontinent.The 1.68Ga metabasalt identified in this study should be the product of the response to the Columbia supercontinent breakup event in the East Kunlun region.The landmasses in western China are likely to participate in global supercontinent evolution as early as the Paleoproterozoic.In the early Caledonian period,under the subduction background of the northern margin of the southern Buqingshan Animaqing Ocean,the limited small ocean basin of the Middle Kunlun Back-arc continued to expand.The Middle Kunlun Ophiolites（502±4.9 Ma）belong to a series of SZZ type ophiolites,which formed in a mature back-arc spreading environment,is a geological record of limited ocean basin expansion in this period.The Middle East Kunlun Ocean in the evolution stage of the Proto-Tethys Ocean should have a bidirectional north-south subduction,forming a series of magmatic rocks with island arc and active continental margin arc properties on both sides of the Kunzhong fault.A series of adakitic rocks represented by the Dulenggou monzogranite（448±5.1 Ma）have a thickened lower crustal origin.They invaded into the oceanic basalt plateau which located in the south of the Middle Kunlun Fault during the Caledonian period,further confirming the southward subduction of the Proto-Tethys Ocean.Longshigeng and Tuolugou are Fe Co and Co Au deposits,which formed by hydrothermal sedimentary mineralization in South Kunlun Belt,respectively.Their metallogenic ages are not earlier than 1032±19 Ma in late Mesoproterozoic and 434±4.6 Ma in Silurian.They formed in the oceanic basalt plateau under the influence of the mantle plume tectonic system and the basin which controlled by the late stage subduction of the Proto-Tethys Ocean,respectively.The ore-forming materials of the hydrothermal sedimentary type cobalt deposits in the East Kunlun are generally from deep mantle sources,wall rock strata or magma.The ore-forming fluids are dominated by exhalative hydrothermal fluid,as well as the addition of vadose hydrothermal fluid or magmatic hydrothermal fluid at the stage of hydrothermal superposition.The multiple sources of ore-forming materials and fluids provide favorable conditions for the migration and enrichment of ore-forming elements.Under the influence of late tectonic processes,the ore body has been subjected to varying degrees of metamorphism,deformation,and transformation:the inversion of strata and ore bodies lead to the occurrence changes,and the horizon extension is limited（Longshigeng）,or after extensive structural replacement,the ore body relocated and shows the characteristics of“layer like but not layer,vein like but not vein”（Tuolugou）.These hydrothermal sedimentary type cobalt deposits has a typical horizon which composed of special lithology—“the dual formation of the hydrothermal sedimentation”.It includes various types of hydrothermal sedimentary rocks and organic-rich black rock series,which constitute the typical"dual formation"of the hydrothermal sedimentation.The hydrothermal sedimentary rocks in Tuolugou are not only siliceous-albite rocks,but siliceous-albite-carbonate rocks with siliceous,albite and carbonatite as different terminal elements,and it also exists transitional compositions.Xiarihamu and Akechukesai are both Ni Co deposits formed by magmatic liquation mineralization in the Central Kunlun Belt.The mineralization was from the Late-Silurian to the Early-Devonian,during which the subduction plate of the Proto-Tethyan oceanic crust was pulled apart and formed a plate window,and the deep asthenosphere material upwelling and pressure reducing melting occurred.Nickel and cobalt in ores are closely related,and cobalt grades are also high in the high nickel grades area in the ore body.The previous estimate in nickel-bearing or non ore-bearing rock is also applicable to cobalt.The ore-bearing rock facies are mainly ultramafic rock facies which rich in olivine and pyroxene.Generally,the higher the degree of partial melting in the mantle source region and the magma differentiation provide good conditions for mineralization.Favorable wall rock conditions（marble,etc.）can make the magma assimilated and contaminated completely,promoting sulfur saturation and further promoting mineralization and Ni Co enrichment.Due to the influence of the distribution coefficient between melts,nickel-sulfides such as nickel pyrite have high cobalt content,pyrrhotite and pyrite contain only a small amount of cobalt,and chalcopyrite basically does not contain cobalt.The magmatic separation type Ni Co deposits in the East Kunlun Belt show different degrees of hydrothermal alteration.The minerals with higher cobalt content are mainly hydrothermal minerals（Ni）cobaltite and gersdorffite,indicating that the hydrothermal process in the late magmatic evolution is of great significance for the local enrichment of cobalt.Dulenggou is a hydrothermal vein type Cu Co deposit which controlled by the faults in South Kunlun Belt,rather than a hydrothermal exhalative sedimentary deposit believed by some scholars.The mineralization age should be in the late Indosinian period under the background of lithospheric detachment.Mantle derived fluids leach and extract the surrounding rock strata during the upwelling process along the fault,resulting in the continuous enrichment of ore-forming minerals in the structurally weak zone.The occurrence,morphology,and scale of the ore body are strictly controlled by NEE trending fault structures,and the ore body has no selectivity for stratigraphic lithology,magmatic rock,etc.,but is greatly related to hydrothermal alteration.The oxidation depth of the ore body is related to the degree of fracture development.The higher degree of fracture development,the deeper oxidation depth of the ore body,and the weaker the oxidation of the ore body away from the fault（deeper from the surface）.On the plane and section,the ore bodies are generally arranged in an oblique array and appear in a lenticular superposition.The deep part of the ore body lies to the east,and the ore-controlling fault shows a"dextral reverse fault"property.The ore body is thick and large in the strike near the east-west direction and in the dip from gentle to steep,showing a segmented enrichment feature.In hydrothermal and hydrothermal sedimentary deposits such as Dulenggou and Tuolugou,cobalt elements occur not only in the form of cobalt pyrite and cobalt bearing pyrite,but also in the form of independent cobalt minerals such as cobaltite（Co As S）,linnaeite（Co₃S₄）,and erythrite（Co₃（As O₄）₂·8H₂O）.The Longshigeng deposit is different from others,with cobalt mainly occurring in the form of oxide——heterogenite（Co O（OH））,closely coexisting with metal oxides dominated by limonite.In magmatic separation type deposits such as Xiarihamu and Akechukesai,cobalt is less likely to occur in silicates,oxides,and sulfide lattices with isomorphism or in sulfides such as nicopyrite and violarite（Ni₂Fe S₄）,but tend to occur in arsenide.The highest content is found in cobaltite and nickel cobaltite（Ni-rich variant of cobaltite）,followed by gersdorffite（Ni As S）and nickel pyrite.Different from the singularity of nickel mineralization,the formation of cobalt deposits is not limited to specific metallogenic geological conditions or factors.It can be enriched and mineralized in various tectonic environments and under various mineralization processes.The genetic types of cobalt deposits in the East Kunlun orogenic belt are diverse,the ore-forming fluids and materials are multi-sourced,and the mineralization is multistage and superimposed.It can be roughly divided into three metallogenic series:hydrothermal sedimentary metallogenic series,magmatic metallogenic series,and hydrothermal metallogenic series.The hydrothermal metallogenic series includes two sub series:hydrothermal vein type and skarn type.During the Precambrian period,under the influence of the mantle plume,the formation of the oceanic basalt plateau of the Wanbaogou Group in the Paleo-Proto-Tethys Ocean laid an important foundation for the later mineralization of the South Kunlun Belt（Longshigeng）;During the Caledonian period,a complete trench-arc-basin system was formed in the southern margin of Qaidam,and hydrothermal sedimentary cobalt mineralization（source beds）occurred in both the South and North Kunlun Belts（Tuolugou and Kendekeke）.Subsequently,the oceanic plate detached and the plate windows were formed,the upwelling of asthenosphere lead to decompression melting.Magmatic separation type Ni Co mineralization occurred in the Central Kunlun belt（Xiarihamu and Akechukesai）;After the closure of Paleo-Tethys Ocean in Indosinian,strong regional magmatic activity and crust-mantle interaction created favorable conditions for the formation of tectonic controlled hydrothermal vein type cobalt deposits（Dulenggou）and skarn type iron polymetallic deposits（Galinge,etc.）.At the same time,the orogenesis in Indosinian also caused the earlier formed deposits to undergo varying degrees of metamorphism and deformation.